Jane's Infantry Weapons hasn't been worth the money in more then a decade, terrible editing and glaring omissions have ruined the books.
Getting a copy of the 2012 Guns Digest will bring you up to speed on the world of civilian guns.
For military firearms the internet is the way to go. Worldguns.ru, manufacturers websites and some of the blogs like firearmsblog.com will get you rolling.
I keep watching ebay for some unknowing fool to put up a recent copy of janes for less than a $200. not that I haven't bought books in that range before, but they were like art works.
I do keep up with the digests, but they are light on things like weights and actual write-ups. Worldguns.ru is a resource I have watched come along slowly over the past few years, and i do find it useful, but IMFDB is just not into the deep info I need.
One thing I always found odd; Okay I have Hogg's Military Small arms of the 20th century, I think it's like the 2003 edition, or something. No shotguns. Seems odd to me. But then again this is also the book without an entry for the M4 Carbine and data for the Canadian C7 and C8 which was comically out dated.
I must mention Paul Mulcahey's Twilight 200 resource site as well
http://www.pmulcahy.com/. The man is epically dedicated and detailed. IE/ a whole SECTION dedicated to the AR15 and another for the Winchester model 70 action. and always working on updating his info, some of which is, yeah innaccurate or out of date. One downside? no pics. Period. Had to do at the time with some issue he had with the owners of the images he was using or somesuch, but still I watch closely for his updates.
Conversely this site
http://smallarmsillustrated.tripod.com/Index.htm; though not IMO well indexed has pics up the wazoo, but no data.